The Reason You Shouldn't Only Use Butter To Caramelize Onions
If your caramelized onions are turning out more burnt rather than actually caramelized, chances are it's because you're cooking them in pure butter. According to Bon Appétit, the best caramelized onions are made with a combination of both butter and oil.
Caramelized onions require slow cooking on the stove over an extended period of time, but butter has a relatively low smoking point. That means it'll burn before your onions have the chance to fully caramelize.
When this happens, Chef Works explains, the onions brown only on the surface, but they haven't technically reached caramelization. Caramelization instead occurs when the natural sugars in the onions get released. It develops a jammy rather than fried consistency, and the color is a deep brown through and through.
To properly achieve this, the addition of oil is necessary. Oil has a high smoking point, so it gives the onions enough time to caramelize without burning. Combined with butter, it makes for caramelized onions with the perfect flavor and texture.
How much butter and oil do you need to caramelize onions?
When caramelizing onions, the ratio of butter to oil depends on how deep of a caramelization you want. Less butterfat yields onions that are softer, while more will allow them to retain their shape, per Bon Appétit. If you're not sure exactly what state of caramelized onions to go for, Chef Works suggests an equal amount of butter and oil.
Just make sure not to exceed the ratio of two tablespoons per two onions, as you don't want to end up frying the onions. You only need enough oil and butter to coat the bottom of the pan without fully submerging the onions.
As per Trial and Eater, once you've measured out your butter and oil, heat them together over medium-low heat, and add the onions. Since the oil is there to prevent the butter from burning, there isn't much monitoring you need to do. Just let the onions cook, stirring occasionally, until they have achieved peak caramelization.